There is presently a need to provide optical communication paths over relatively long distances on the order of hundreds, or even thousands, of kilometers. Providing such a long-haul communications path often proves challenging. The maximum distance at which optical communications are feasible, and the maximum signal bit rate, are limited by the frequency stability of the semiconductor laser used to generate an optical signal. If the optical signal exhibits frequency chirp, i.e., frequency shifts over time, the signal will be degraded as it traverses the length of a fiber optic cable. These degradations, which include chromatic dispersion and modal dispersion, become more pronounced as the length of cable is increased, and/or as the bit rates are increased, until, at some point, it is no longer possible to recover intelligible information from the optical signal. Therefore, there is a need to minimize frequency chirp in optical communications systems.
In optical communications systems, Y-branched waveguides have been employed to provide digital optical switching and optical signal modulation. A typical Y-branch digital optical switch is designed such that two waveguide branches intersect at a very small angle to form a Y-shaped structure. The composition of the waveguide structure may include any of a wide variety of materials, such as lithium niobate (LiNbO.sub.3), and/or various semiconductor materials. One example of a Y-branch digital optical switch is described by M. N. Khan in the 1995 ECOC Proceedings, Vol. 1, pages 103-106. Another Y-branch switch is disclosed in U.S. Pat. No. 5,594,818, entitled, "Digital Optical Switch and Modulator and a Method for Digital Optical Switching and Modulation", issued on Jan. 14, 1997 to Edmond J. Murphy.
Most existing methods of operating optical signal modulators that use the Y-branch configuration change the refractive indices of both output waveguide branches. A modulated signal generated in this manner suffers from frequency chirp. In order to modulate an optical signal, the light propagation direction in one of the waveguide branches is changed by forcing a refractive index change in one of the branches with respect to the other branch. In the aforementioned Murphy patent, this is accomplished by imposing a biasing voltage across the two output waveguide branches. Therefore, the refractive indices in both waveguide branches will change in this instance. In an adiabatic Y-branch modulator, the direction of light propagation follows the waveguide branch having the highest refractive index. Note that, as used herein, the term adiabatic refers to processes involving continuous evolution as opposed to abrupt transitions.
Although many Y-branch modulators induce changes in the refractive indices of both output waveguide branches by applying a voltage to the branches, it should be noted that such changes could also be induced by applying current and/or other external forces to selected sections of both output waveguide branches. However, irrespective of the type of force that is used to induce refractive index changes, all existing proposed methods of controlling Y-branch modulators provide a modulated signal that exhibits frequency chirp.
What would be desirable would be the capability to selectively control the amount of frequency chirp produced by a Y-branch modulator.